Digital halftoning refers to any algorithmic process that creates the illusion of continuous-tone images from the judicious arrangement of binary picture elements (pixels).
RIPing (Raster Image Processing) is the digital image processing technique that takes a continuous-tone image and original graphical elements and provides a binary halftoned output image file. RIPing an image file usually can include the steps of spatial filtering, interpolation, tone and color modification, and digital halftoning.
There is a need to control the size of the dots in a pre-existing halftone bitmap. This bitmap could have been generated by a digital screening system (i.e., a RIP) or it could have been generated by scanning an optical piece of film to create a digital bitmap file. In general, these bitmap files were created with some output device in mind. As such, the dot pattern was created to produce a given density to dot percentage relationship tailored to the target output device. If this bitmap file were printed on an output device that had different dot-gain characteristics than the target device the density as a function of dot-area percentage will be different. Thus, in order to achieve the desired dot percentage to output density relationship of the target device, on a different device, the bitmap file for the target device must be modified according to the dot-gain differences between the target device and the given output device.
For most traditional digital-proofing scenarios this dot-gain compensation is done by re-RIPing the original art with a modified dot-gain curve. This modified dot-gain curve accounts for the dot-gain differences between the target device and the proofing system. Thus, a custom bitmap file needs to be generated for each output device. The main drawback to this paradigm is that the proofs may be made on systems that do not use the same screening technology as was used to create the target bitmap file. Thus, in order to support a RIP-once-output-many (ROOM) workflow the dot-gain compensation needs to be applied to the bitmap file directly.
Bitmap files can consist of single channel bitmaps such as those for a single color device (e.g., a black-and-white laser printer), those for a four-color (e.g., CMYK) output device (e.g., a printing press or a graphic-arts proofer), or any output device that utilizes bitmap files of any number of channels (e.g., a multi-ink inkjet printer).
Several approaches for dot-gain modification of bitmaps have been proposed. The dot gain may be adjusted for each of the primary colors cyan, magenta, yellow, and black. A description for how to do this is disclosed by Spence in U.S. Pat. Nos. 5,255,085 and 5,293,539 titled “ADAPTIVE TECHNIQUE FOR PROVIDING ACCURATE TONE REPRODUCTION CONTROL IN AN IMAGING SYSTEM” and “METHOD AND APPARATUS FOR CALIBRATING TONE REPRODUCTION IN A PROOFING SYSTEM”. Here, percent dot area is calculated using the Murray and Davies equation from measured densities.
Denber, et. al. disclose a method of shifting and ANDING a bitmap image with itself to thin the image displayed in U.S. Pat. No. 5,250,934 “METHOD AND APPARATUS FOR THINNING PRINTED IMAGES” and also teaches a method of setting a bit to an intermediate level if it is diagonally between two active bits using shifting, logical AND, and a logical OR operation.
Mailloux, et al. discloses using a 4×4 input to a lookup table to determine how to operate on the central 2×2 pixels to implement halfbit or fullbit dilation and erosion in U.S. Pat. No. 5,483,351 titled “DILATION OF IMAGES WITHOUT RESOLUTION CONVERSION TO COMPENSATE FOR PRINTER CHARACTERISTICS”. This requires knowing some of the surrounding pixels when deciding how to dilate or erode the pixels in the center.
Eschbach teaches in U.S. Pat. No. 5,258,854 titled “CONVERTING BETWEEN WRITE-WHITE, WRITE-BLACK, AND NEUTRAL BITMAPS”, how to resize bitmap images in small amounts less than one full bit in size. Eschbach states that the erosion and dilation may be by differing amounts in the x and y directions, and that the amount of resizing may be a fraction of a full pixel.
Loce, et al. teaches logically combining two morphological filter pairs and an original image to create an output image in U.S. Pat. No. 5,680,485 titled “METHOD AND APPARATUS EMPLOYING EROSION-BASED FILTER PAIRS FOR IMAGE MAPPING”. The morphological filters described are erosion filters, one of which has less erosion than desired and the other having more erosion than desired. Logically combining the original image with two eroded images provides for a method of obtaining an intermediate result.
Eschbach describes a method of resizing an input bitmap in U.S. Pat. No. 5,208,871 titled “PIXEL QUANTIZATION WITH ADAPTIVE ERROR DIFFUSION”. Eschbach simulates a scan of an output image from an input bitmap such that the scan resolution is different from the input bitmap. Error diffusion is utilized to quantize the output bitmap into the desired output bit resolution. This example uses error diffusion to spread out the error in the quantization of a multilevel pixel into a reduced number of output states.
U.S. Pat. No. 6,115,140, title “Method and System for Half Tone Color Conversion”, inventors Bresler and Nosko, issued Sep. 5, 2000 teaches using a descreened version of an original image, dilated and eroded versions of the original image to select a combination of the original, dilated, and eroded image to effect a dot gain or tone scale change in an input bitmap image. The patent discloses an original halftone image, an eroded version (HE), and two dilated versions (HD1 and HD2). Then a weight based on descreened versions of the original halftone (CO), the color corrected original (CI), the eroded original (CE), and the two dilated originals (CD1 and CD2) is calculated. The descreened images are used to select which of the four halftone images are transferred into H1 and H2. The weighting function is then used to merge bitmap versions of H1 and H2 together into the tone scaled output bitmap (HO). How to descreen is not disclosed, nor exactly how to calculate which bit of H1 and H2 is used to drive the output bit HO. The need to use error diffusion to distribute the error in selecting between H1 and H2 is not mentioned. Dilation is described as growing a single pixel completely around the halftone feature. A second dilation grows two pixels completely around the halftone feature. Similarly, erosion subtracts a single pixel completely around the halftone feature. This patent fails to teach how to perform descreening.
U.S. Pat. No. 4,630,125, titled UNSCREENING OF STORED DIGITAL HALFTONE IMAGES, inventor Roetling also states that “A partial solution known in the art is to spatially filter the halftone image with a low pass filter.” Roetling teaches that the spatial filter method is not an exact method as it tends to blur the original image.